Brief Description of Related Developments
The present invention relates to antenna systems in communication devices, and in particular to fault isolation in antenna systems.
Nowadays radio telephones are being increasingly installed in motor vehicles, either retrofitted by the customer himself or already fitted when the vehicles are produced, which vehicles can be, for example, motor vehicles or any other vehicles with radio communications devices, for example ships, aircraft, trains, etc. Thus, it is possible, for example, to install two antenna paths for GSM (Global System for Mobile Communications) antennas which can operate as normal antennas or emergency antennas. When the antennas are installed, or during later operation of the vehicles, there is, however, the risk of the respective coaxial lines, the plugs or the antennas themselves being damaged. Satisfactory communication by means of these antennas is then possible only with difficulty, or no longer possible at all. Corresponding problems may also occur in antenna branches of Global Positioning Systems (GPS).
DE 44 34 915 C1 has already disclosed a method and an arrangement for determining the incorrect matching of a load connected to a signal source, in particular of an antenna connected to a transmitter. In the known method, a power which is to be radiated is fed into the antenna branch, a power which is reflected in the antenna branch (by extraction of a portion of the power) is sensed and the extracted portions of the fed-in power and the reflected power are fed to an evaluation means. For this purpose, the two powers are compared in order to obtain the quality signal which indicates the quality level of the antenna branch.
It is thus possible to determine whether or not an antenna branch is defective. If it is, it must be replaced, which is relatively time-consuming and cost-intensive.
The invention is based on the object of specifying a method and a communications device which can be used to determine the position of a defective area in the antenna branch relatively precisely.
A method according to the invention for determining the position of a defective area which is present in an electrical line path and has signal-reflecting properties comprises the following steps: a signal which has a starting frequency is coupled into the line path or antenna path; the amplitude of a standing wave is measured in absolute terms at a fixed angular position of the oscillation by means of a detector, the standing wave being generated between the defective area and the detector as a result of reflection of the coupled-in signal at the defective area; the frequency of the coupled-in signal is changed in one direction until, after at least just 180xc2x0 the detector detects again at the aforesaid angular position a corresponding amplitude absolute value of the standing wave which is being formed; and a reading which indicates how often the detector has detected this amplitude absolute value is used, together with the respectively associated frequency and the starting frequency, to determine a fault interval between the detector and defective area.
In this context, the fault interval Ed can preferably be determined by means of the following equation:
Ed=X(c/fo)
Where
X=n/2xc3x971/[(fn/fo)xe2x88x921]
Here, Ed is the fault interval, n is the reading having the values 1, 2, 3 etc., f0 is the starting frequency, fn with the values n=1, 2, 3, etc. the frequency of the coupled-in signal for the reading n=1, 2, 3, etc., and c is the propagation velocity of the signal in the line path. Here, c is preferably calculated by means of the expression   c  =      1                                        μ            ⁢            0            ⁢            ϵ            ⁢            0            ⁢            ϵ                    ⁢                      xe2x80x83                    ⁢          r                ⁢                  xe2x80x83                    
Here, xcexc0 is the permeability constant, ∈0 is the dielectric constant and xcex5r is the relative dielectric constant of the line path.
Since the attenuation of the waves propagating along the line path depends on the frequency, according to one advantageous development of the invention standardized amplitude absolute values are measured by the detector. In order to form the standardized amplitude absolute values, the amplitudes of the radiated-in and reflected waves are compared with another, that is to say standing wave ratios are formed.
In principle, it is possible to acquire the respective amplitude absolute values at any angular position of the full oscillation, that is to say over the range 2xcfx80 or 360xc2x0. Thus, for example, the amplitude absolute values can be measured in the region of the minima of the standing wave or in the region of the maxima of the standing wave. This makes measurement of the respective amplitude absolute values considerably easier. This also makes the circuitry design simpler.
The frequency of the coupled-in signal is preferably changed continuously starting from the starting frequency, as a result of which the position of defective areas along the line path can be determined even more precisely. However, frequency jumps could also be carried out, but these must not be too large. In the process, the frequency of the coupled-in signal can, depending on requirements, either be increased or decreased starting from the initial frequency.
If an antenna path of a radio telephone is used as the line path, the frequencies used are preferably those on which the radio telephone can transmit. The frequencies may be those of the GSM band. A separate test device for testing the antenna path can thus be dispensed with.
The method itself can, for example, be carried out after the antenna path has been installed in a vehicle or motor vehicle using the radio telephone as a transmitter, that is to say in the last fabrication step of the communications device. However, it is also possible to carry out the method periodically or whenever the radio telephone is put into service, in order to ensure that the communications device has ready status even over a relatively long time period.
A communications device according to the invention is composed of an antenna path installed in a vehicle, and a radio telephone which can be connected to the antenna path. The radio telephone contains the following: a transmitter for coupling a signal having a starting frequency into the antenna path; a detector for measuring in absolute terms the amplitude of a standing wave at a fixed angular position, the standing wave being formed between the detector and a defective area present in the line path; a control device such as for example a programmable microprocessor 10, for changing the frequency of the coupled-in signal in one direction until after at least 180xc2x0 the detector detects again at the aforesaid angular position a corresponding amplitude absolute value of the standing wave which is formed; a counter within the control device 10 which counts how often the detector has detected this amplitude value; and a computing device for calculating the interval between the defective area and the detector by reference to the starting frequency, to a reading of the counter and to the frequency associated with the respective reading, of the coupled-in signal.